In general, plastic containers are widely used for various purposes because of good formability and low production cost. One problem with such plastic containers was that when the containers contain a viscous material such as mayonnaise-like food products, it is hard to use up all the contents as the material tends to stick to the inner wall surface of the container. In this respect, coating materials that improve the ability of the contained material to slide down have been developed recently, and it has been known that with such a coating material being applied on the inner wall surface of the container, the contents in the container can easily be used up due to the improved ability to slide down the inner wall surface of the container.
In order for such a coating material to favorably exhibit its properties, it is necessary to apply the coating material uniformly on the inner wall surface of the container. However, plastic containers come in various shapes and have a small opening diameter, and with the use of a spray gun (see, for example, PTL 1) commonly used in an application apparatus, it was difficult to apply the coating material uniformly on the inner wall surface of the container.
As one method of applying the coating material uniformly on the inner wall surface of the container, the spray gun could be inserted in the container, and a spray of coating material could be ejected while the spray gun is rotated and moved up and down or back and forth. Alternatively, with the spray gun inserted in the container, the container could be rotated instead of the spray gun, and the coating material could be ejected while the spray gun is moved up and down or back and forth.